High temperature turbine rotor shafts and method of heat treating



United States Patent Office 3,1 10,636 Patented Nov. 12, 1053 This invention relates to high temperature turbine rotor shafts and to the heat treatment thereof. More particularly, it relates to a process for the differential heat treatment of high temperature steam turbine rotor shafts and shafts produced thereby.

In modern steam turbines, the steam inlet end of the rotor shaft is often subjected to temperatures ofl000 F. and upward while the opposite exhaust end may be at room temperature. It will be quite evident to those skilled in the art that ideally the high temperature end of such a shaft should be possessed of desirable long-time, high temperature properties, such as high creep strength, rupture strength, metallurgical stability, ductility and the like, whereas the lower temperature end should have high strength and toughness for resistance to bursting. According to prior art practice, such turbine rotors are subjected as a whole to that heat treatment which will produce the best balance between the properties desired at the high and low temperature ends.

A primary purpose of this invention is to provide rotors for high temperature steam turbines which are differentially heat treated and a method for differentially heat treating such rotors so that the optimum high temperature characteristics may be developed at one end of the rotor along with desirable low temperature characteristics at the opposite end of the rotor.

Briefly, according to the present invention high temperature turbine rotors having specific compositions amenable to such treatment are differentially heat treated with respect to their length to develop at one end favorable high temperature properties and at the other end desirable low temperature properties.

Those features of the invention which are believed to be novel are set forth with particularity in the claims appended hereto. The invention will, however, be better understood from a consideration of the following descriptron.

It is desirable that steam turbine rotor shafts, the ends of which operate at widely different temperatures, have at the colder end good fracture toughness or a low Charpy transition temperature, while at the same time the other end which is heated to elevated temperatures has good high temperature strength. in order to produce in a large mass of steel not responsive to immersion quenching, such as a turbine rotor shaft, a low Charpy transition temperature, such alloy must have a low bainite start temperature in order to produce alow temperature transformation product and, in general, be heat treated to pro duce a fine grain size. Normally, the production of a fine grain size requires low austenitizing temperatures which also tend to produce low creep rupture strength as opposed to the high creep rupture strengths required at high temperatures. Conversely, in order'to produce good high temperature strength, the alloy must contain refractory carbides, such as those from the group consisting of molybdenum, tungsten, vanadium, tantalum, columbium, titanium or zirconium. In general, these elements require high austenitizing temperatures in order to provide a beneficial high temperature strength. A general class of steel which meets this requirement of being capable under low austenitizing temperatures to produce a fine grain material, and under high austenitizing temperatures to produce high temperature strength, is that having the general weight percent composition shown in the following table:

Table I Range Preferred 0.025 max. 0.025 max. Remainder.

Remainder" Shown in column 3 of the table is a specific steel from within this class which has been demonstrated to respond satisfactorily to this type of heat treatment. It will be realized, of course, that the above compositions may be changed in some ways Without detracting from their present benefits. For example, molybdenum and tungsten may be interchanged as equivalents. Also, additional manganese and nickel can be substituted for some of the chromium. Somewhat higher vanadium contents and small niobium additions to those shown may also be beneficial in some respects. Likewise, materials such as tantalum and zirconium can be substituted on an equivalent basis for such materials as molybdenum, tungsten, vanadium, eolumbium and titanium.

In carrying out the present invention, the rotor is heated preferably in a vertical furnace so that the high temperature end is held at a temperature of from about 1800 F. to 2000 F. and the cold end at a temperature F. to 200 F. lower than the high temperature end for a period of from about 20 to 4-0 hours. After completion of this part of the heat treatment, the rotor is cooled as by means of circulated cold air generally over a period of about eight hours so that the forging is lowered to a temperature below about 600 F. and above about 300 F. and is then equalized at a temperature of 300 F. to 600 F. for from about 12 to 24 hours. A water spray quench may be used as an alternate method of cooling or in combination with the air cooling. Following such equalization of temperature, the temperature is raised at a rate of about 60 F. per hour until a temperature of about 1000 F. to 1300 F. is reached, the rotor being held at such temperature for about twelve to sixty hours and the furnace cooled. A temperature gradient of up to 100 F. may be required to produce the desired hardness level over the length of the forging, because the difference in austenitizing temperature between the two ends affects their response to softening in the final treatment. If desired properties are not attained with the first temper, additional suitable tempering at up to about 1300.F. can be carried out.

The following example will illustrate the practice of the invention.

A rotor having a maximum diameter of 47 and a length of 18' and weighing approximately 24 tons, having the composition shown in the third column of the above table, 'WaS heated in a differential vertical furnace which maintained the high temperature end of the rotor at 1850 F. and the low temperature end at 1650 F. for 23 hours. The forging was then removed from the furnace, placed in a cooling fixture and cooled while rotating at a speed of about 2 rpm. by compressed air from two 3000 cim. blowers directed against the rotory body. The rotor was so cooled [for about eight hours at which time the cooler end was at a temperature of about 350 F. and the hotter end at a temperature of about 550 F. The

rotor was then charged into a 500 F. furnace, equalized for 12 hours, raised at normal rates to a temperature of 1150 F. at the low temperature end and =1200 F- at the high temperature end, held 48.5 hours and the fur nace cooled. It was then reheated to a uniform temperature of 1200 F., held 47.5 hours and furnace cooled. Specimens taken from the hot and cold ends of the rotor (surface radial specimens) so heat treated had physical characteristics as shown in Table 11 below.

From the above it will be quite apparent that rotor shafts treated according to the present invention are characterized by desirable high temperature characteristics at the end which is exposed to high temperature motive fluid, whereas the end exposed to the cooler exhaust fluid is characterized by favorable low or room temperature characteristics. For example, the hot end of the rotor is possessed of good 1000 F. rupture strength and ductility whereas the cold end has a desirable low fracture appearance transition temperature or resistance to bursting along with the small grain size which accompanies such characteristic and high strength. it will also be seen that other properties, such as tensile strength, yield strength, percent elongation 'and reduction in area are similar for the two ends of the rotor shaft.

There are provided, then, by the present invention high temperature steam turbine rotor shafts and methods for preparing such shafts, the ends of which may be subjected respectively to elevated temperatures and low temperatures, the respective ends of the rotor having been heat treated to develop therein the maximum favorable physical characteristics desirable at the operating temperatures claimed.

What I claim as new and desire to secure by Letters lPatent of the United States is:

1. The method of producing a high temperature turbine rotor shaft having salutary high temperature properties at one end thereof including relatively high rupture strength and ductibility and desirable. low temperature properties at the other end thereof including a relatively r low fracture appearance transition temperature which comprises differentially heat-treating a shaft having a weight percent composition of carbon 0.40 maximum, manganese 1.5 maximum, silicon 0.50 maximum, nickel 2.0 maximum, chromium 2.0 to 5.0, vanadium 0.1 to 1.0, niobium 1.0 maximum, titanium 0.5 maximum, molybdenum 0.5 to 2.0, tungsten 2.0 maximum, phosphorus 0.05 maximum, sulfur 0.05 maximum, with the remainder essentially iron, for from 20 to 40 hours to a temperature of 1800 F to 2000 F. at one end and from about 100 F. to 200 F., lower at the other end, cooling over a period of about 8 hours to 600 F., equalizing for from aboutlZ to 24 hours at a temperature of 300 F. to 600 F., heating to a temperature of 1000 F. to 1300 F. for about 12 to 60 hours and furnace cooling.

2. The method of producing a high temperature turbine rotor shaft having salutary high temperature properties at one end thereof including relatively high rupture strength and ductility and desirable low temperature properties at the other end thereof including a relatively low F fracture appearance transitiontemperature which comprises difierentially heat-treating a shaft having a weight percent composition of carbon 0.40 maximum, manganese 1.5 maximum, silicon 0.50 maximum, nickel 2.0 maximum, chromium 2.0 .to 5.0,. vanadium 0.1 to 1.0,

niobium 1.0 maximum, titanium 0.5 maximum, molybdenum 0.5 to 2.0, tungsten 2.0 maximum, phosphorus 0.05 maximum, sulfur 0.05 maximum, with the remainder essentially iron, for from 20 to 40 hours to a temperature of 1800 'F. to 2000 F. at one end and from about F. to 200 F., lower at the other end, cooling over a period of about 8 hours to 600 F., equalizing for from about 12 to 24 hours at a temperature of 300 F. to 600 F., heating to a temperature of 1000 F. to 1300 F. for about 12 to 60 hours and furnace cooling while maintaining a temperature differential of about 100 F. higher at the hot operating end.

3. The method of producing a high temperature turbine rotor shaft having salutary high temperature properties at one end thereof including relatively high rupture strength and ductility and desirable low temperature properties at the other end thereof including a relatively low fracture appearance transition temperature which comprises differentially heat-treating a shaft having a weight percent composition of carbon 0.40 maximum, manganese 1.5 maximum, silicon 0.50 maximum, nickel 2.0 maximum, chromium 2.0 to 5.0, vanadium 0.1 to 1.0, niobium 1.0 maximum, titanium 0.5 maxi-mum, molybdenum 0.5 to 2.0, tungsten 2.0 maximum, phosphorus 0.05 maximum, sulfur 0.05 maximum, with the remain der essentially iron, for from 20 to 40 hours to a temperature of 1800 F. to 2000 at one end and from about 100 F. to 200 F., lower at the other end, water spray cooling over a period of about 8 hours to 600 F, equalizing for from about 12 to 24 hours at a temperature of 300 F. to 600 F., heating to a temperature of 1000 to 1300 F. for about 12 to 60 hours and furnace cooling.

4. The method of producing a high temperature turbine rotor shaft having salutary high temperature properties at one end thereof including a relatively high rupture strength and ductility and desirable low temperature properties at the other end thereof including a relative low fracture appearance transition temperature which comprises differentially heat-treating a shaft having a :weight percent composition of carbon 0.40 maximum, manganese 1.5 maximum, silicon 0.50 maximum, nickel 2.0 maximum, chromium 2.0 to 5.0, vanadium 0.1 to 1.0, niobium 1.0 maximum, titanium 0.5 maximum, molybdenum 0.5 to 2.0, tungsten 2.0 maximum, phosphorus 0.05 maximum, sulfur 0.05 maximum, with the remainder essentially iron, for from 20 to 40 hours to a temperature of 1800 F. to 2000 F. at one end and from about 100 F. to 200 F., lower at the other end, air cooling over a period of about 8 hours to 600 F., equalizing for from about 12 to 24 hours at a temperature of 300 F. to 600 F, heating toa temperature of 1000 F. to 1300 F. for about 12 to 60 hours and furnace cooling.

5. The method of producing a high temperature turbine rotor shaft having salutary high temperature properties at one end thereof including relatively high rupture strength and ductility and desirable low temperature properties at the other end thereof including a relatively low fracture appearance transition temperature which comprises differentially heat-treating a shaft having a weight percent composition of carbon 0.40 maximum, manganese 1.5 maximum, silicon 0.50 maximum, nickel 2.0 maximum, chromium 2.0 to 5.0, vanadium 0.1 to 1.0 niobium 1.0 maximum, titanium 0.5 maximum, molybdenum 0.5 to 2.0, tungsten 2.0 maximum, phosphorus 0.05 maximum, sulfur 0.05 maximum, with the remainder essentially iron, for from 20 to 40 hours, to a temperature of 1800 to 2000 F. at one end and from about 100 F. to 200 F, lower at the other end, air and water spray cooling over a period of about 8 hours to 600 F., equalizing for from about 12 to 24 hours at a temperature of 300 F. to 600 F, heating to a temperature of 1000 F. to 1300 F. for about 12 to 60 hours, and furnace cool- 6. The method of producing a high temperature turbine rotor shaft having desirable high temperature properties at one end thereof including relatively high r-upture strength and ductility and desirable low temperature properties at the other end thereof including a relatively low fracture appearance transition temperature, which comprises heat-treating a shaft having a weight percent composition of carbon 0.24 to 0.34, manganese 1.0 maximum, silicon 0.15 to 0.35 maximum, nickel 0.5 maximum, chromium 3.0 to 4.0, vanadium 0.15 to 0.40, molybdenum 1.0 to 1.5, phosphorus 0.025 maximum, sul fur 0.025 maximum, with the remainder essentially iron, for about 23 hours at a temperature of 1850 F. at one end and 1650" F. at the other end, cooling over a period of about eight hours to a temperature of about 550 F. at the hot end and 350 F. at the cool end, equalizing for about twelve hours at a temperature of 500 F., tempering at a temperature of about 1200 R, and furnace cooling.

7. The heat-treated high temperature turbine rotor shaft produced by the process of claim 4.

References Cited in the file of this patent UNITED STATES PATENTS 1,333,767 Napier Mar. 16, 1920 2,202,759 Denneen et a1 May 28, 1940 2,831,789 Gorman Apr. 22, 1958 3,044,872 Hayes et al. July 17, 1962 FOREIGN PATENTS 35,198 Netherlands Nov. 15, 1934 

1. THE METHOD OF PRODUCING A HIGH TEMPERATURE TURBINE ROTOR SHAFT HAVING SALUTARY HIGH TEMPERATURE PROPERTIES AT ONE END THEREOF INCLUDING RELATIVELY HIGH RUPTURE STRENGTH AND DUCTIBILITY AND DESIRABLE LOW TEMPERATURE PROPERTIES AT THE OTHER END THEREOF INCLUDING A RELATIVELY LOW FRRACTURE APPEARANCE TRANSITION TEMPERATURE WHICH COMPRISES DIFFERENTIALLY HEAT-TREATING A SHAFT HAVING A WEIGHT PERCENT COMPOSITION OF CARBON 0.40 MAXIMUM, MANGANESE 1.5 MAXIMUM, SILICON 0.50 MAXIMUM, NICKEL 2.0 MAXIMUM, CHROMIUM 2.0 TO 5.0, VANADIUM 0.1 TO 1.0, NOBIUM 1.0 MAXIMUM, TITANIUM 0.5 MAXIMUM, MOLYBDENUM 0.5 TO 2.0, TUNGSTEN 2.0 MAXIMUM, PHOSPHORUS 0.05 MAXIMUM, SULFUR 0.05 MAXIMUM, WITH THE REMAINDER ESSENTIALLY IRON, FOR FROM 20 TO 40 HOURS TO A TEMPERATURE OF 1800*F. TO 2000*F. AT ONE END AND FROM ABOUT 100*F. TO 200*F., LOWER AT THE OTHER END, COOLING OVER A PERIOD OF ABOUT 8 HOURS TO 600*F., EQUALIZING FOR FROM ABOUT 12 TO 24 HOURS AT A TEMPERATURE OF 300*F. TO 600*F., HEATING TO A TEMPERATURE OF 1000*F. TO 13//*F. FOR ABOUT 12 TO 60 HOURS AND FURNACE COOLING. 